FIG. 5 is a sectional view of a conventional speaker. In FIG. 5, the conventional speaker includes voice coil body 504 with coil part 503 movably disposed in magnetic gap 502 formed by magnetic circuit body 501; diaphragm 508 connected to the outer circumferential surface of voice coil bobbin 505 of voice coil body 504 and connected to frame 507 through edge 506; and corrugated damper 509 placed closer to magnetic circuit body 501 than from diaphragm 508, connected to the outer circumferential surface of voice coil bobbin 505 and to frame 507.
Passing an AC current with a sound signal added thereto through coil part 503 of voice coil body 504 causes voice coil body 504 to vibrate vertically. The vertical vibration of voice coil body 504 is transmitted to diaphragm 508, which vibrates air to reproduce sound.
In such a conventional speaker, damper 509 supports voice coil body 504 together with a complex of diaphragm 508 and edge 506, and follows vibration of voice coil body 504 by means of the corrugated structure expanding and contracting during sound reproduction. Information on prior art documents related to the patent application includes FIG. 8 in patent literature 1.
With the conventional speaker, the Young's modulus of damper 109 needs to be high (rigid) to some extent in order to support voice coil body 104. With the high Young's modulus of damper 509, vertical amplitude of voice coil body 504 higher than a certain level during sound reproduction rapidly increases the movable load, which makes it difficult for voice coil body 504 to vibrate faithfully according to an input signal. Consequently, the speaker creates distortion during sound reproduction.
FIG. 6 is a sectional view of another conventional speaker. In FIG. 6, the speaker includes voice coil body 603 with the coil part movably disposed in magnetic gap 602 formed by magnetic circuit body 601; diaphragm 606 connected to the outer circumferential surface of the voice coil bobbin of voice coil body 603 and connected to frame 605 through first edge 604; and damper 608 placed closer to magnetic circuit body 601 than from voice coil body 603, connected to the side surface of voice coil body 603, and connected to frame 605 through second edge 607. First edge 604 projects in the direction opposite to that of second edge 607 to make vertical amplitude of diaphragm 606 symmetric, thereby reducing distortion with the speaker. Here, information on prior art documents related to the patent application includes patent literature 2.
With the above-described speaker, in order to reduce distortion created during sound reproduction, it is important to make vertical amplitude of diaphragm 606 symmetric, and the Young's modulus of first edge 604 is preferably nearly equal to that of a complex formed of damper 8 and second edge 607.
Here, first edge 604 and a complex formed of damper 608 and second edge 607 need enough rigidity to support voice coil body 603 and have a high Young's modulus, and thus first edge 604 unlikely deforms following vertical amplitude of voice coil body 603. Consequently, diaphragm 606 connected to first edge 604 deforms during sound reproduction, causing disturbance in frequency response in the middle and high ranges.
Under the circumstances, reducing the Young's modulus of first edge 604 requires reducing that of the complex formed of damper 608 and second edge 607 as well. This is because the Young's modulus of first edge 604 is preferably nearly equal to that of the complex.
However, first edge 604 and the complex formed of damper 608 and second edge 607, with their Young's moduli thus reduced, are unable to well support voice coil body 603.    [Patent literature 1] Japanese Patent Unexamined Publication No. H11-150791    [Patent literature 2] Japanese Patent Unexamined Publication No. 2007-88674